1. Field of the Invention
The present invention relates to desalination of water, and more specifically to a system & method for efficient and low energy desalination of water, based on borderline fast fluctuation between liquid to gaseous state and back.
2. Background
According to the World Watch 2000 report we are depleting the planet's water resources at the rate of 109 billion gallons of water per day. Many areas in the world already suffer shortages of water, and others will suffer from it in the coming years. Israel, for example, is now in a critical stage of water shortage, with the Kineret sea's water level already at a critically low level. Therefore more efficient water sweetening is essential for our survival on this planet.
The most commonly used water sweetening methods are: Reversed osmosis, distillation, electrodyalisis, and partial freezing. However, all of these methods suffer from low efficiency and high energy consumption, thus making them a number of times more expensive than naturally obtained water, which is one of the main reasons why they are not sufficiently implemented yet despite the general water shortages.
A new and much more efficient direction, based on a fast fluctuation on the borderline between liquid and gaseous states, has been described in a few recent patents: U.S. Pat. No. 4,323,424, issued on Apr. 6, 1982 to Secunda et. al., Spanish patent ES 2018732, issued on May 1, 1991 to Lumbreras & Gimenez, and U.S. Pat. No. 5,207,928, issued on May 4, 1993 to E. J. Lerner, describe methods for generating with compressed air, without heating, a stream of high velocity saltwater droplets, which for a split second vaporize because of the temporary vacuum created on their trail, allowing the salt to precipitate and fall, and then immediately condense again. These short-lived fluctuations are the key to much higher potential efficiency, because no extreme conditions of temperature or pressure are needed. However, mixing the water with compressed air enters too much air into the process, which limits the efficiency.
Also, these patents used too small droplets and with such droplets the salt particles that are created are too small to fall down and can mix again with the water. The 1982 patent, which was apparently the first to go in this direction, used droplet size of up to 6 micron, apparently didn't understand the real nature of the process, and was designed mainly for extracting the salt, without being able to properly extract also properly sweetened water.
European patent application number WO0110526, by Aquasonics Corp., which quotes the above mentioned patents, describes a better process, in which, without air pressure, salty water is injected though an array of vertical nozzles (with internal diameter of about 0.75–1.23 mm each) by a pressure of approx. 10 atmospheres, creating water droplets with sizes of about 30–100 micron each and sonic speeds of about 300 meters per seconds, which then evaporate, allowing the salt to fall down between the upright nozzles, and then immediately recondense. According to the inventors, this process allows 95% efficiency in the recovery of fresh water and separation of salts, compared to about 36% for other processes, so that the process is altogether about 3 times more efficient than other current methods and is about 4 times cheaper, both in terms of setup costs and operating costs. So instead of the usual price of about $2–2.5 per thousand gallons of sweetened water, the Aquasonics process is estimated to cost about $0.66 per thousand gallons—which is Approximately 0.70 Israeli Shekels per Kub, which thus makes it more or less equal for example to the price of natural water in Israel.
However, even the above better process still has a number of limitations: 1. Such small nozzles can easily become clogged by salt or other small particles such as for example sand, etc. 2. Since the salt falls down on the area of the nozzles, they have to be sufficiently apart from each other in order to give sufficient room to the salt to fall between them, and also the salt is actually falling over the path of newly injected droplets, so it can mix with some of them. 3. Creating a high pressure of about 10 Atmospheres, although of course much better than about 50 Atmospheres needed for reversed Osmosis, still consumes considerable energy, and normal pressure pumps have only limited efficiency, so only part of the energy goes to the actual speeding of the droplets. A better process that doesn't have these problems would be very desirable, since it could increase efficiency even further and reduce costs to even cheaper levels.
However, the 1982 (Secunda et. al.) patent was not limited to the use of air pressure—it mentioned for example also that “the spray of droplets of required size may be produced by forcing the liquid under pressure through a small aperture”. It also used in one of the embodiments a rapidly rotating small cup of about 4 inch diameter on a horizontal axis of rotation. However, it used salty water solutions with at least 15% or more salt, which enabled the resulting salt particles to become large enough even with an initial water droplet size of a few microns. On the other hand, according to the tests conducted by Secunda et. al., increasing the initial water droplets size to tens of microns or more considerably increases the time needed for the droplets to evaporate—for example a water droplet at the size of 5 micron can evaporate in about 10 ms (milliseconds), but increasing the size to 30 microns can cause the evaporation time to go up to near 1 second or even more. This would be very problematic, since upon impact with the air the droplets quickly slow down, so after 1 meter they already move much slower or stop. But even if they didn't slow down before the end of the 1st meter, since at a speed of 300 meters per second the droplets travel 1 meter in 3.3 ms, they would have to evaporate in less than 3 ms. However, according to Secunda et. al., a 20 micron diameter sphere injected into still air at the speed of sound would come to rest in about 7 cm and in a time of about 4 ms. And smaller spheres would stop even quicker. So if the above measurements and calculations of Secunda et. al. are correct, the process can work properly only with higher salt concentrations. When the salt concentration is lower as for example about 3% in sea water, water droplet size of a few microns would produce salt particles too small to fall down, and a water droplet with a size of 30 microns or more would require too much evaporation time. Therefore, better solutions are needed in order to solve this contradiction of parameters for desalination of sea water.
However, there is even a bigger problem with the above data—and that is the energy requirements. A physical energy calculation shows that in order to accelerate 1 Kub of water to 300 meters per second we need 12 Kilowatt-Hour of electricity, so at a cost of 7 cents per Kilowatt-hour, the mere energy requirements are 84 cents per Kub, and that is assuming that we have a 100% efficiency in recovering the desalinated water and no energy losses on the way. So of course the real costs are bigger. On the other hand, electricity costs vary a according to the time of day, etc., so the price can come to half if operated at night for example. The root of the problem is that in reverse osmosis a large percent of the energy of the high pressured water is recovered and reused, whereas the Kinetic energy of the flying droplets is lost.